Electrochemical Activation of LiGaO2: Implications for Ga-Doped Garnet Solid Electrolytes in Li-Metal Batteries

Ga-doped Li7La3Zr2O12 garnet solid electrolytes exhibit the highest Li-ion conductivities among the oxide-type garnet-structured solid electrolytes, but instabilities toward Li metal hamper their practical application. The instabilities have been assigned to direct chemical reactions between LiGaO2 coexisting phases and Li metal by several groups previously. Yet, the understanding of the role of LiGaO2 in the electrochemical cell and its electrochemical properties is still lacking. Here, we are investigating the electrochemical properties of LiGaO2 through electrochemical tests in galvanostatic cells versus Li metal and complementary ex situ studies via confocal Raman microscopy, quantitative phase analysis based on powder X-ray diffraction, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and electron energy loss spectroscopy. The results demonstrate considerable and surprising electrochemical activity, with high reversibility. A three-stage reaction mechanism is derived, including reversible electrochemical reactions that lead to the formation of highly electronically conducting products. The results have considerable implications for the use of Ga-doped Li7La3Zr2O12 electrolytes in all-solid-state Li-metal battery applications and raise the need for advanced materials engineering to realize Ga-doped Li7La3Zr2O12for practical use.


■ INTRODUCTION
Garnet-structured solid electrolytes enable all-solid-state Limetal batteries (ASSLMBs) to have the potential to enhance the safety and energy density of ASSLMBs significantly in comparison to conventional Li-ion batteries (LIB). 1−3 Among the family of garnet-structured solid electrolytes derived from Li 7 La 3 Zr 2 O 12 (LLZO), Ga-doped LLZO (Ga:LLZO) shows the highest Li-ion conductivity at room temperature, exceeding 1 mS cm −1 . 1,4,5Ga:LLZO thus receives a lot of interest and is intensely studied for its relation of crystal chemistries, structures and ionic conductivities, 6−11 structural stabilization for optimizing its conductivities, 12,13 sintering optimization for creating dense ceramics with high purity, 7,14−16 thin film deposition for the application as thin film electrolyte, 17 and application as a thick film for solid-state battery fabrications. 4,5,13,14In general, it has been found that optimized conductivities >1 mS cm −1 at room temperature can be achieved through the stabilization of the cubic garnet phase at low levels of Ga-doping, that is, 0.2 < x < 0.3 in Li 7−3x Ga x La 3 Zr 2 O 12 , while 6.4 < Li < 6.6. 6,11,15,17or higher Ga substitutions (x > 0.5 mol per formula unit of LLZO), β-LiGaO 2 emerges as an obvious secondary phase in Ga:LLZO that can be detected by X-ray diffraction 7,9,10 and at the grain boundaries of the garnet phase by electron microscopy. 6However, studies employing transmission and electron microscopy have recently demonstrated that even at considerably low Ga-doping levels (<0.1 mol per formula unit of LLZO), LiGaO 2 can be found at grain boundaries and in triple junctions in the densified Ga:LLZO ceramic pellets. 4,5,13t the same time, it has been reported that LiGaO 2 accumulation on the grain boundary can be correlated to the instability of Ga:LLZO toward Li metal. 4,5,13For example, Li et al. reported severe cracking once Ga:LLZO pellet is in contact with molten Li. 13 In an attempt to study the reaction mechanism, LiGaO 2 was reacted with liquid Li metal, where Li 2 Ga and Li 2 O formation was observed.The huge volume expansion from these educts to products was thus identified as the cause of the microstructural cracking to collapse LiGaO 2 containing Ga:LLZO pellets when in contact with molten Li metal. 13arious efforts were undertaken to control and eliminate the presence of LiGaO 2 phases in Ga:LLZO to improve the stability of Ga:LLZO against Li metal effectively.Su et al.  found that the presence of LiGaO 2 is related to abnormal grain growth of LLZO particles during sintering. 16They developed a processing strategy via a two-step sintering leading to fine grains of Ga:LLZO after the sintering process and thus avoiding LiGaO 2 secondary phase formation to improve the stability against Li metal. 16Li et al. reported to control and suppress the secondary LiGaO 2 phase formation by adding 2 wt % SiO 2 into Ga:LLZO powder during the sintering process. 13This resulted in improved thermomechanical and electrochemical stability when in contact with molten Li.It is hypothesized that SiO 2 extracts Li from LiGaO 2 to form Li 2 SiO 3 while the remaining Ga incorporates into the LLZO bulk. 13n one of our previous studies, we compared and analyzed t h e r e a c t i o n s b e t w e e n L i m e t a l a n d s i n t e r e d Li 6.4 Ga 0.2 La 3 Zr 2 O 12 and Li 6.45 Ga 0.05 La 3 Zr 1.6 Ta 0.4 O 12 , in comparison to Ga-free Li 6.45 Al 0.05 La 3 Zr 1.6 Ta 0.4 O 12 (Ta:LLZO).We demonstrated that Ga:LLZO undergoes chemical reactions with Li metal, while Ta:LLZO remains stable.In this study, we could also demonstrate that Ga from the Ga:LLZO bulk forms Li−Ga alloys at the grain boundaries of the Ga:LLZO ceramic after reaction with molten Li.By NMR measurements, it was proven that Ga-ions can leach out from the bulk into the grain boundary after contacting Li metal. 4,5The primary observation of Ga leaching into the grain boundaries of the Ga:LLZO ceramic questions the actual role of LiGaO 2 in the failure mechanism of the Ga:LLZO solid electrolyte.
In the literature, β-LiGaO 2 is well-known in the area of semiconductor research and semiconductor industry, where it serves as a substrate for GaN growth. 18β-LiGaO 2 itself can be grown into rather large single crystals, 18,19 and its crystal structure was determined by Marezio in 1964 to be identical to that of orthorhombic β-NaFeO 2 . 20Therefore, its structural, optical, and electrical properties are well understood, 21−24 but it is hard to extrapolate, which role LiGaO 2 may play in the instability of Ga:LLZO toward Li, even in an isolated environment without physical contact to Li.Furthermore, its stability and reactions under a larger electrochemical voltage, such as in a full battery cell, are not yet understood.Here, we investigate LiGaO 2 from an electrochemical point of view to understand the possible impact of β-LiGaO 2 in Ga:LLZO ceramics under dynamic conditions, i.e., under an applied electrical field and electrochemical cycling conditions, as in a battery.To accomplish this, we synthesize β-LiGaO 2 and examine it as the positive electrode in a galvanic cell for understanding its electrochemical properties.The results contribute to a better understanding of the Ga:LLZO failure mechanism when in contact with Li metal, which may offer a better strategy for fabricating and processing the Ga:LLZO solid electrolyte for ASSLMB application.

■ EXPERIMENTAL SECTION
LiGaO 2 Synthesis and Electrode Preparation.An adapted version of the Ga:LLZO synthesis procedure was used to produce LiGaO 2 powder. 4The synthesis was performed by mixing Li 2 CO 3 and Ga 2 O 3 powders in an autogrinder (Retsch) in a 1:1 ratio, adding a slight excess of Li 2 CO 3 (10 wt %) to compensate for Li-evaporation losses.The powder mixture was heated to a temperature of 1200 °C with a heating ramp of 5 K min −1 in a muffle furnace in air in a closed corundum crucible.The reaction product was cooled freely after 8 h and reground in the autogrinder to produce a fine powdered sample.
To test the powder for its electrochemical activity, it was processed by a typical laboratory-scale procedure into an electrode tape with a 10:10:80 ratio of polyvinylidene difluoride (PvDF, Alfa-Aesar): carbon black: LiGaO 2 . 13Prior to the slurry preparation, carbon black powder (Super P, Alfa-Aesar) and LiGaO 2 powder were dried in a vacuum furnace at 80 °C for 16 h.The powder was mixed and ground in a mortar for more than 20 min.The mixture was added to polyvinylidene difluoride (PvDF), dissolved in N-methylpyrrolidone (NMP, Alfa-Aesar), and stirred vigorously for 5 h.The slurry was drop-coated on a ⌀11 mm Ni current collector and dried for at least 24 h in a hood, following another 24 h in a vacuum oven at 80 °C.
The final electrode has a mass loading of ∼5 mg, as determined for every sample by weighting the nickel plate before coating and the nickel plate plus coating individually.
Electrochemical Measurements.The as-prepared LiGaO 2 electrodes were tested as positive electrodes against Li metal using a glass fiber separator (Whatman GF/D) and a polypropylene separator saturated with 1 M LiPF 6 in an ethylene carbonate and dimethyl carbonate (vol 1:1) electrolyte (LP30, battery grade, Sigma-Aldrich).The cells were assembled in Swagelok cells.Each cycling protocol started with a 20 h open circuit voltage (OCV).Afterward, cyclic voltammetry (CV) and galvanostatic charge/discharge experiments were carried out.The CV was recorded with a scanning rate of 0.02 mV s −1 .Three different potential ranges are applied in the CV: 0.6−4.2V vs Li/Li + , 0.05−3 V vs Li/Li + , and 0.05−4.5 V vs Li/Li + .At the upper and lower cutoff potentials, the potential was held for 20 h each.The same CV protocol was applied to initialize and monitor each cell for one cycle before the cells were measured through galvanostatic charge−discharge.The galvanostatic charge/discharge tests were performed at a current density of 10 mA g −1 in a potential range from 0.05 to 4.5 V vs Li/Li + .At the respective cutoff potential, the potential was held for another 20 h for constant potential charge and discharge, respectively.To monitor the cell's equilibrium state afterwards, OCV was applied for 20 h before the next charge or discharge started.At least 7 cycles were recorded.
To prepare samples for the ex situ measurements, battery cells are assembled as described above and brought to the desired potential within the first CV cycle at a scanning rate of 0.02 mV/s.At each desired potential, a constant potential period of 20 h was applied, followed by an OCV period until the potential equilibrated.Table 1 gives an overview of the investigated ex situ samples: A-1 (for the OCV potential after equilibration of the just assembled cell), B-1 (4.5 V vs Li/Li + ), C-1 (0.05 V vs Li/Li + ), and D-1 (2.75 V vs Li/Li).Furthermore, ex situ samples were taken in the last (7th) cycle of the charge−discharge measurements at the upper and lower cutoff potential labeled as B-7 (4.5 V vs Li/Li + ) and C-7 (0.05 V vs Li/Li + ).
X-Ray Powder Diffraction.X-Ray powder diffraction (XRPD) was used to measure the LiGaO 2 powder after synthesis and the cycled ex situ samples (A-1, B-1, C-1, D-1, B-7, and C-7).The LiGaO 2 powder sample was prepared by using the "front-load" method.A lab diffractometer (Empyrean, Panalytical) with a copper anode (Kα wavelength: 1.54 Å) in reflection geometry was used to measure the LiGaO 2 pristine powder from 10 to 120°2θ, with a counting time of 30 s per step and a step size of 0.008°2θ.The ex situ samples were taken from the disassembled cells in a glovebox, mounted on Si substrates with Scotch Magic tape, and placed in a sample holder.The tape can be well penetrated by X-rays but protects the sample from direct contact with the atmosphere during the measurement. 25A lab diffractometer (D8 Discover, Bruker) with a copper anode (Kα wavelength: 1.54 Å) in reflection geometry was used to measure the ex situ samples in an angular range from 10 to 120°2θ, with a counting time of 20 s per step and a step size of 0.02°2θ.Structural analysis and quantitative phase analysis (QPA) based on the XRPD data were carried out within the software package Diffrac.Topas (Bruker).The LiGaO 2 powder sample was structurally analyzed according to the Rietveld method. 26The ex situ samples were analyzed by QPA from a full pattern fit of the XRPD data. 27icro-Raman Spectroscopy.Micro-Raman spectroscopy was carried out with a WITEC alpha300R microscope using a solid-state 532 nm excitation laser and 600 L/mm grating with a laser power of 10 mW.The Raman spectra were collected with a 100× objective on an area of 80 μm × 80 μm and a 0.5 μm step raster for the pristine and ex situ samples of the first cycle.This yields 160 × 160 pixels, each containing an individual Raman spectrum.Due to the very low Raman activities and huge topography of the ex situ samples in the seventh cycle, the Raman analysis was carried out via volume mapping (by changing the stage height, i.e., focus point) in a volume of 50 μm × 50 μm × 50 μm with a 1 μm step size, yielding 50 × 50 × 50 pixels containing each one individual Raman spectrum.Each individual measurement at a point (pixel) was carried out with a 1 s acquisition time for both the volume and area mappings.The collected scans were treated by correction algorithms for cosmic ray removal, noise filtering, and baseline correction.Afterward, the data sets were analyzed through principal component analysis within the WITEC software package "WITEC project".
Scanning and Electron Microscopy and Energy-Dispersive X-Ray Spectroscopy.Both, top views (A-1, B-1, and C-1 and B-7) and cross-section analyses (B-7) were carried out to investigate the microstructure and elemental composition via scanning and electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX).The samples were consistently managed under vacuum or argon, and the Swagelok cell disassembly took place within an argon atmosphere inside a glovebox (MBraun and GS).
For top-view measurements, samples were investigated without further treatment, with the sample attached onto the SEM sample holder by carbon tape.For the cross-section preparation, the samples, including the Ni current collector, were divided into quarters using a Buehler IsoMet low-speed saw operating at a rotational speed of approximately 30 rpm.Subsequently, one of the quarters was ground using 3 M diamond sandpaper on a JEOL.A handy-lap grinder, starting with a grain size of 15 μm, progressing through 9, 3, 1, and 0.5 μm.Following this process, the ground surface underwent crosssectional polishing in a JEOL IB-19530 CCP, utilizing an accelerating voltage of 4 kV.Active LN2 cooling to −100 °C was applied to prevent the temperature elevation of the sample.During the crosssectional polishing, approximately 100 μm of the material was removed.The cross-section sample was affixed onto an SEM sample holder at a 90°angle, ensuring the polished side faced upward.
SEM images were collected on a Quanta FEG 650 FEI, USA, using an accelerating voltage of 5−20.0 kV with a secondary electron detector.Additionally, an FEI Helios NanoLab 460F1 FIB-SEM was used for a concentric back-scattered (CBS) image for the crosssectional sample.For the EDX-analysis, an Ametek Octane Super at an accelerating voltage of 20 kV was employed on the top-view sample and an Ametek Octane Elite Super at an accelerating voltage of 5 kV for the cross-sectional sample.
X-Ray Photoelectron Spectroscopy and Electron Energy Loss Spectroscopy.Synchrotronic XPS measurement for LiGaO 2 at seventh cycle (charged state) and metallic Ga (as a reference) were performed in a 14A beamline of the Taiwan light source.Cryogenic electron energy loss spectroscopy (Cryo-EELS) measurements were conducted using an FEI Titan G2 ChemiSTEM 80−200 transmission electron microscope (TEM) equipped with a high-brightness field emission gun and probe spherical aberration (Cs) correction system operated at 200 kV.To mitigate electron beam damage, the sample was characterized by using a cryo-transfer holder (Simple Origin, Model 200).The EEL spectra were acquired in scanning transmission electron microscopy (STEM) mode with an annular dark-field (ADF) detector.The convergence semiangle was ∼25 mrad.■ RESULTS AND DISCUSSION Structural Characterization of LiGaO 2 Powder.Figure 1 presents the X-ray powder diffraction pattern of the synthesized LiGaO 2 .The observed Bragg peaks can be assigned to a reference structure of β-LiGaO 2 in space group Pna2 1 (ICSD coll.code 18152). 20This phase was also used as a starting model in the following Rietveld analysis.The fit was carried out considering the LiGaO 2 scale factor, sample displacement, size, and strain broadening and through refining all relevant structural parameters, such as lattice parameters, Debye−Waller factors, fractional coordinates, and occupancies.All structural parameters (except Li fractional coordinates and Li isotropic Debye−Waller factors) were freed to converge.Their final results are given in Table 2.The lattice parameters only slightly deviate from the model structure (deviation less than 0.1%), and fractional coordinates only slightly shift compared to the model structure (shift less than 2%).There are no secondary phases visible in the pattern.Hence, our synthesis produced phase pure β-LiGaO 2 via solid-state reaction at 1200 °C in the space group Pna2 1 .We are thus reporting the identical compound of LiGaO 2 in space group Pna2 1 , which has been reported in the literature and has been extensively studied for its crystal structure, 20,28 band structure, 29,30 application as a substrate for GaN thin film deposition, 18,19,31 and employment as a dielectric ceramic. 32urthermore, the synthesized LiGaO 2 polymorph is expected to be identical to the LiGaO 2 secondary phases in the Ga:LLZ system that have been reported in the  literature. 7,9,10It is known that, besides the polymorph in spacegroup Pna2 1 , three other polymorphs exist for LiGaO 2 .These polymorphs crystallize in either the α-NaFeO 2 structure type (spacegroup R3̅ m), 33 the NaCl structure type (spacegroup Fm3̅ m), 22 or in the β-LiFeO 2 structure type (spacegroup I4/m). 22In the literature, the LiGaO 2 secondary phases were mostly identified from the main characteristic feature at 21− 23°2θ in XRPD.Only LiGaO 2 in spacegroup Pna2 1 possesses this characteristic feature, due to its two main reflections 1 1 0 and 0 1 1 at 21.66 and 22.38°2θ, respectively.The other reported polymorphs are either high-pressure phases or metastable and thus unlikely to be formed during the reaction of Ga:LLZ at high temperature (>950 °C) and ambient pressure. 7,9,10Thus, we expect that the herein reported LiGaO 2 polymorph in space group Pna2 1 is the very same polymorph that has frequently been reported to coexist as a secondary phase with Ga:LLZ garnet. 7,9,10lectrochemical Analysis of LiGaO 2 Electrodes.The as-synthesized LiGaO 2 powder was processed into electrodes by a typical procedure to be tested against Li metal in a laboratory cell. 34,35Prior to each CV scan, the cells were set to the OCV for 20 h to reach their equilibrium potential between 2.3 and 2.5 V vs Li/Li + .Figure 2 displays the results of the electrochemical activities during the CV scans in different electrochemical potential windows.In the first tested electrochemical potential window (Figure 2a), from 0.6 to 4.2 V vs Li/Li + , a small nonreversible reduction reaction was observed at ∼0.8 V vs Li/Li + .In the second tested electrochemical potential window (Figure 2b), from 0.05 to 3.0 V vs Li/Li + , the CV scan discloses a more significant activity in the lower electrochemical potential region below 0.6 V vs Li/Li + , peaking −20 mA g −1 upon reduction at 0.05 V vs Li/Li + .The reaction shows reversibility but degrades quickly.The third tested electrochemical potential window is also the widest window tested, reaching from 0.05 to 4.5 V vs Li/Li + (Figure 2c).The CV records additional activities above 3.0 V vs Li/Li + upon oxidation and a dramatically increased activity below 1 V vs Li/ Li + , peaking at −70 mA g −1 in the first cycle upon reduction.The reactions display reversibility, although incomplete, as can be seen from the absence of symmetries in the oxidation and reduction curves.A large degradation of activity is observed upon cycling, leading to a fade of the peak current at 0.05 V vs Li/Li + during reduction to less than −10 mA g −1 in the seventh cycle.
The results demonstrate that the activity of LiGaO 2 strongly depends on the choice of the lower and upper cutoff potential.The recorded activities above 3 V vs Li/Li + may be critical for the access of the full activity in the low potential region, as can be concluded from the comparison of cycling to 3.0 and 4.5 V (Figure 2b,c).The displayed behavior in the CV gives a strong hint that a combination of high-voltage decomposition (or activation) and subsequent reversible reactions lead to a significant electrochemical activity through LiGaO 2 decomposition products in the potential window from 0.05 to 1.0 V vs Li/Li + .
To investigate the electrochemical behavior deeper, a dedicated charge−discharge cycling protocol was applied to freshly prepared cells.The cycling protocol applied an initial OCV and 1 CV cycle followed by constant current (CC) and constant potential (CP) charge and discharge in the potential range from 0.05 to 4.5 V vs Li/Li + (Figure 3a).After each CC and CP dis/charge, an OCV of 20 h was applied to allow for the potential equilibration. Figure 3b demonstrates the achieved CC capacities and voltage profiles over 7 cycles.Like the CV results, the reaction activity is mostly concentrated at lower potentials <1.5 V vs Li/Li + .Also, a reversible reaction is demonstrated over several cycles, with significant degradation of the achieved CC capacities.
Figure 3c displays the achieved CC charge and discharge capacities over cycle number and compares them with the achieved CC+CP discharge capacities over cycle number.It is obvious that applying a CP step after the CC steps yields 25− 55 mAh g −1 additional charge capacity and 70−115 mAh g −1 additional discharge capacities.In the first cycle, a CC+CP charge capacity of 42 mAh g −1 is recorded, followed by a discharge capacity of 269 mAh g −1 and a second charge capacity of 172 mAh g −1 .In the seventh cycle, 109 and 106 mAh g −1 are recorded upon charge and discharge, respectively.In general, the CP charge and discharge capacities tend to get larger with a higher cycle number, while the CC+CP charge capacities first rise and then fade, whereas the discharge capacities show a steady fade.The corresponding efficiencies as a function of the cycle number are displayed in Figure 3d.Very large CC and CC+CP efficiencies are recorded for the first cycle: the discharge capacities are 1195% (CC) and 640% (CC +CP) higher than the charge capacities in the first cycle.In the second cycle, they fall to 111% (CC) and 136% (CC+CP) and fade slowly to 51% (CC) and 97% (CC+CP) in the seventh cycle.
If all Li can be extracted from LiGaO 2 during charging, then it would have a theoretical capacity of 247 mAh g −1 .The first charge capacity (42 mAh g −1 ) thus suggests that up to 17% of the total amount of Li in the LiGaO 2 electrode has been extracted and transported to the negative electrode upon releasing one electron per extracted Li into the outer circuit during the CC+CP charge.As indicated by the high discharge capacity of the first cycle (269 mAh g −1 ), 640% more Li is brought back into the positive electrode during the following discharge than was taken out during charge.This is only possible if a reaction product with a high Li storage capacity has formed upon charging in the first cycle.In the further cycles, this compound may undergo reversible electrochemical reactions with Li, as can be concluded from the reversible nature of the displayed activities in CV and galvanostatic charge−discharge cycling.Furthermore, the decomposition of LiGaO 2 into materials that allow to reversibly react to high amount of Li seems to be ongoing, at least until the sixth cycle, because of the recorded efficiencies above 100%.
During each OCV period between CC+CP charge and discharge, the evolution of the cell potential was recorded for20 h.As can be seen in Figure 3a, there is a significant potential drop in each OCV period after charging to 4.5 V vs Li/Li + and a noticeable potential rise during each OCV period after discharging to 0.05 V vs Li/Li + .After the first charge, the potential falls from 4.5 to 2.9 V vs Li/Li + during 20 h of the OCV period.The potential after this 20 h OCV does not reach the equilibrium potential yet, as an asymptotic behavior of the potential-overtime function is not yet displayed.After the first discharge, the potential value rises from 0.05 to 0.2 V vs Li/Li + during 20 h of OCV.Here, an asymptotic behavior is displayed, and the cell seems to be in equilibrium.Figure 3e displays the (pseudo) equilibrated potential values at the end of each OCV period.As can be seen, the potentials after discharge rise slightly after the individual cycle, from 0.20 to 0.24 V vs Li/Li + .On the other hand, the recorded potentials at the end of a 20 h OCV after charge show a continuous decrease over cycle number from 2.9 V vs Li/Li + to 1.45 V vs Li/Li + .
The potential equilibrations to higher and lower potentials during an OCV period after charge and discharge, respectively, are typical for electrochemical cells due to polarizations and diffusion limitations.The potential will naturally equilibrate with the specific electrode potential of the material at the specific state of charge.Slower reaction kinetics during charge or discharge will usually lead to a stronger potential drop during OCV.The potential rise from 0.05 to 0.2 V vs Li/Li + in the OCV period after the first discharging thus suggests that the reaction has not been completed and a compound with an electrochemical potential of 0.2 V vs Li/Li + is formed.Most likely, the reaction would have kept going, if a smaller discharge current and a longer CP period had been applied.This is also supported by the observed decrease of the current during the CP period, which does not show full completion of the asymptotic trend yet after 20 h.This means the compound that we observe at the end of discharge is only partially electrochemically reacted, which is important information for interpretation of the later ex situ experiments.Furthermore, the slight rise in electrochemical potential during the individual OCV periods after each discharge suggests that the reaction kinetics worsen during cycling, meaning that compounds with different discharge states are produced at the end of each discharge cycle.
The potential drop in the OCV period after charging is significant and implies an additional underlying mechanism in addition to limited reaction kinetics.After the first charge, the potential during OCV drops back to a value the same as the pristine cell (2.95 V vs Li/Li + ), even though a significant capacity had been recorded.The dominant mechanism here seems to be the decomposition of LiGaO 2 above 4 V vs Li/Li + .
Obviously, the decomposition product must have a lower electrochemical potential than LiGaO 2 , which is why the cell tends to equilibrate back to the electrochemical potential of LiGaO 2 during the OCV periods after discharge (which has the highest electrochemical potential in the formed composite material).Interestingly, the recorded potentials at the end of each OCV period decrease to 2.1 V vs Li/Li + in the fifth cycle and then drop to 1.41 and 1.45 V vs Li/Li + in the sixth and seventh cycles.The significant drop of the level of OCV means that LiGaO 2 must have either completely reacted into a different compound or has been isolated within the electrochemical cell in the sixth and seventh cycles, i.e., through microstructural degradation and loss of contact between LiGaO 2 and the reactions products, binders, and carbon.
Even though significant capacity degradation was observed, the underlying reversible nature in the following cycles strongly suggests that the formed product is a compound with a high reversible capacity in the lower potential range <1.5 V vs Li/ Li + .−38 Structural and Microstructural Ex Situ Analyses.To further understand the electrochemical reactions of LiGaO 2 , ex situ analyses via SEM, XRPD, and micro-Raman spectroscopy were carried out.Figure 4a indicates the measured voltages of the ex situ samples, which are also listed in Table 1.A-1 was taken after the initial OCV for potential equilibration, which has an equilibrated potential of 2.0 V vs Li/Li + .B-1 was taken after oxidation through CV scan to 4.5 V vs Li/Li + and then set to OCV for 20 h.The final equilibrated potential of the cell was 3.0 V vs Li/Li + .C-1 was taken after oxidation to 4.5 V and reduction to 0.05 V vs Li/Li + and resting at 0.05 V vs Li/Li +  The results from Raman microscopy are shown in Figure 4b.Interestingly, the averaged Raman spectra all show the very same features and not much deviation from each other.As such, the recorded spectra for the pristine sample but also all the ex situ samples agree well with the Raman spectrum of β-LiGaO 2 .β-LiGaO 2 has point group C 2v with experimental confirmed bands at 128.7 cm −1 (A 1 (1) ), 204.2 cm −1 (B 1 (1) ), 252.1 cm −1 (A 1 (2) ), 289 cm −1 (A 2 (1) ), 444.3 cm −1 (A 1 (3) ), 493.3 cm −1 (A 1 (4) ), 502.1 cm −1 (A 1 (5) ), 643.9 cm −1 (A 1 (6) ), and 653.8 cm −1 (A 1 (7) ). 28,39Accordingly, the Raman spectra for the pristine sample and ex situ A-1, B-1; C-1, D-1, and C-7 were indexed to the bands of β-LiGaO 2 in point group C 2v .Only the spectrum for the B-7 sample cannot be assigned to β-LiGaO 2 alone.Here, the bands from the β-LiGaO 2 modes seem to have vanished while new, weak, and broad bands emerged at 99, 145, and 182 cm −1 (indicated by * in Figure 4b), which will be discussed in more detail later.
Similarly, the XRPD patterns (Figure 4c) show LiGaO 2 in Pna2 1 , as the main phase in all ex situ samples, resembling the pristine powder.There are changes in the intensity ratios of the two main reflections, and shoulders emerge predominantly at the 0 1 1 reflection at 22.6°(Figure S1 in the Supporting Information provides a more detailed view).Further, it is noted that the reflection maximum intensities get weaker, and the Bragg peaks tend to broaden throughout the sample series.The peak broadening suggests a significant change in a microstructure, i.e., crystallite sizes or strains of LiGaO 2 crystallites.At this stage, it must be assumed that any possible decomposition products are hard to detect through Raman spectroscopy and XRPD where their only evidence may lay in the subtle changes in the spectra (e.g., ex situ sample B-7) and their diffraction patterns (e.g., intensity ratios and peak shoulders in B-1 to C-7).
As the Bragg peak broadening is the most obvious from samples A-1 to B-1, electron microscopy was used as a complementary tool for understanding the underlaying mechanism.A drastic change in the microstructure was observed in the SEM images for the sample A-1 to the sample B-1 (Figure 4d,e) where sample A-1 shows LiGaO 2 particles with a size distribution from 1 to 10 μm, with most of the particles in between 7 and 10 μm and sample B-2 on the other hand shows smaller particle distribution between 1 and 5 μm and only a few particles above 7 μm.These results suggest that, even though LiGaO 2 seems to be majorly preserved in the ex situ samples, it does undergo a significant microstructural change.For sample C-1 (Figure 4f), a new microstructural feature is present, which has the appearance similar to a melt.Similarly, optical images taken for all ex situ samples show a drastic changing microstructure from a pure powder-like sample to a solidified specimen under the presence of a melt phase (see Figure S2, Supporting Information).
Quantitative Phase Analysis from XRPD.The possible presence of Li−Ga alloys or Ga was further analyzed through quantitative phase analysis based on the collected XRPD data.In parallel, the LiGaO 2 phase was quantified with respect to its lattice parameters and crystallite sizes to understand its contribution to the reactions during cycling.To describe the main LiGaO 2 phase in the diffractograms, the refined structural parameters of the LiGaO 2 powder sample (Figure 1, Table 2) were used as the starting values.Parameters like the fractional coordinates, Debye−Waller factors, and site occupancies were kept fixed at their refined values from the LiGaO 2 powder sample, while lattice parameters, sizes, and strains were refined against the measured data.Besides LiGaO 2 and Ni (as the CC), possible impurity phases such as Li 3 N, Li 2 O, LiOH, LiOH•H 2 O, and Li 2 CO 3 and a series of possible reaction products such as Ga 2 O 3 , LiGa 5 O 8 , Li 5 GaO 4 as well as various Li−Ga alloys, and Ga metal were testes within the quantitative phase analysis routine.It was concluded that none of the above-mentioned phases could be fitted reasonably against the measured data besides Ni, Li 3 N, and some specific compositions of a Li−Ga alloy.−43 Figure 5a shows the fitting results of QPA for the sample series.The evolution of the refined lattice parameters and crystallite sizes is shown in Figure 5b,c.For comparison, the refined lattice parameters and crystallite size for the pristine LiGaO 2 powder sample are shown, too.As can be seen, the refined values for the lattice parameters remain constant throughout the sample series, while the crystallite sizes show a clear decreasing trend upon cycling.The crystallite size for the ex situ A-1 (OCV equilibrated) sample refines to a similar value (820 nm) than the refined crystallite size for the pristine powder sample (823 nm).For the B-1 sample, the crystallite size refined significantly smaller to 108 nm.Over the sample series, this value steadily decreases and refined to 77 nm for C-7.
The XRD patterns for C-1, D-1, B-7 and C-7 cannot be described from the contribution of a LiGaO 2 phase alone.A small fraction of Li 3 N contaminations must be considered, which most likely arises from reactions with the ambient atmosphere.Even though the sample was protected from the atmosphere by the Scotch magic tape, these reactions can happen if highly reactive phases (such as Li or Li−Ga alloys) are present in the sample.Neither Ga 2 O 3 nor Li−Ga−O related phases, such as LiGa 5 O 8 or Li 5 GaO 4 could be fitted against the measured data.Still, C-1, D-1, B-7, and C-7 remained poorly described by considering only LiGaO 2 and Li 3 N.This is especially evident from the region between 21 and 22°2θ, where shoulders and peaks next to the main reflection of LiGaO 2 emerge (see Figure 5a, enlarged region).
For ex situ C-1 and C-7, the main reflection of Li 5 Ga agrees with the position of the shoulder at 22.5°2θ.However, considering its contribution to the diffraction pattern of the C-1 sample, is successful only if a high texture for its 0 0 l reflection is considered.Similarly, LiGa 6 can be fitted against the measured data for D-1 and B-7 and successfully describe the shoulders and peaks and 22.0−22.5°2θ,if a strong texture for its 0 0 l reflections is considered.While this may indicate the presence of Li−Ga alloys in the sample, it cannot completely exclude the presence of Ga, Ga 2 O 3 , or Li−Ga−O ternary compounds in the samples.Ga metal, which has a melting point at 29.77 °C, is most likely in the liquid state or a supercooled liquid and thus X-ray amorphous.On the other hand, Li−Ga-oxides or Ga 2 O 3 , if decomposed from LiGaO 2 at higher voltage, are unlikely to show a good crystallinity.They may present as a low-crystalline or nanoscale secondary phase that cannot readily be identified in the given diffraction patterns.
Component Analyses from Micro-Raman Spectroscopy Mapping.To investigate possible reaction products in the system further, the collected micro-Raman maps for the samples B-1 (1st charge), D-1 (1st full cycle), and B-7 (7th charge) were analyzed through a principal component algorithm within the WITEC project software package.The results of the analysis are displayed in the color-coded maps in Figure 6a−c, and their representative individual spectra of the identified components are shown in Figure 6d−h.After the first charge and after the first full cycle, the main component in the mapped area was LiGaO 2 (Figure 6d), as was already represented by the averaged spectrum from the mapped area of the sample B-1 and D-1 in Figure 4b.
In the mapped area of the seventh charge to 4.5 V vs Li/Li + , a new main component (Figure 6e) appears to be widely distributed over the mapped area.The component shares the feature of the averaged spectra of sample B-7 in Figure 4b, with a main, yet weak, band at 99 cm −1 and 2 more weak bands at 145 and 182 cm −1 .Compared to LiGaO 2 , the newly identified component has a much lower Raman activity, as can be seen from the absolute intensity scales in Figure 6d,e.From the electrochemical analysis, where similarities with the Ga 2 O 3 and Ga electrochemical activities vs Li were recorded, decomposition products such as gallium oxides or Ga metal may be expected.−47 Most of the experimentally recorded Raman spectra for Ga 2 O 3 do not agree to the observed features for the identified component in Figure 6e.Similarly, not a single band matches the Ga-metal Raman spectrum. 48The only similarities are identified for the κ/ε-Ga 2 O 3 spectrum. 47However, due to the low Raman activity and poor signal/noise, the assignment of bands is rather difficult.Besides, in the spectrum, we note that some features of the LiGaO 2 spectrum are still well preserved, like the A 1 (4) and A 1 (5) modes as the main feature in the LiGaO 2 spectrum at ∼490 cm −1 .Furthermore, LiGaO 2 components with their characteristic spectra have been identified in some areas of the sample dominantly, as shown  by the red areas in the map of sample B-7.Three secondary components with a small distribution in the mapped area and a rather weak intensity compared to that of the collected LiGaO 2 spectrum were identified (Figure 6f−h).The results suggest that, even at the scale of micro-Raman spectroscopy (lateral resolution of 500 nm), single phases cannot be identified.Most likely, the sample is heavily inhomogeneous even at the upper nanoscale.Yet, the presence of Li−Ga alloys or liquid Ga cannot be excluded from being present in the sample because of their Raman inactivity.
Scanning Electron Microscopy and Energy-Dispersive X-Ray Spectroscopy.As a further step to identify the nature of the possible decomposition and reaction products, SEM and correlative EDX mappings were collected for the B-7 sample (Figure 7).In the top view of the sample, the binder in the electrode and carbon additive can still be clearly identified from its F−K and P−K signals.Carbon seems to be majorly present in the selected area, as evidenced from its C−K signal.However, Ga or O failed to be localized and showed only a weak and broad distribution throughout the investigated area.
The cross-section EDX map reveals a high concentration of C at the surface of the electrode sheet.F−K and P−K signals can be found throughout the electrode layer and indicate the presence of the used binder.Toward the current collector, the microstructure turns from a densified top layer into an agglomeration-like structure with obvious pores and microstructural disintegration of material from the rest of the layer.Yet, the gap between the layer and current collector may be an artifact from the cross-section polishing procedure.Ga can be localized in densified agglomerates, in addition  8d, there is no peak belonging to metallic Ga in Li 1s spectra, which was used as a reference.It is interesting to observe the peak ratio difference of Ga−O (1119.65 eV) to the metallic gallium (Ga−Ga) peak (1116.53eV). 52,59The Ga−Ga peak is comparably smaller in the B-7 sample than in the Ga reference; however, it indicates the presence of metallic gallium for the LiGaO 2 electrode in the charged state after the seventh cycle, which will be further discussed later.similarities in Ga−O binding energies but could also not be separated from LiGaO 2 at the length scale of SEM/EDX.To probe an even smaller length scale and decouple LiGaO 2 form possible Ga 2 O 3 signals, cryo-EELS measurements were conducted on the B-7 sample (charged to 4.5 V after the seventh cycle).A low-loss EELS spectrum from the bulk of this sample indicates the absence of Li in this area (Figure 9b).
The first cycle starts from the OCV at 2.75 V vs Li/Li + .In the first charge, above 3.5 V versus Li/Li + , LiGaO 2 is activated, meaning it starts to decompose.Li + transfer from the positive LiGaO 2 electrode to the negative Li-metal electrode is evident from the measured current.Here, Li + is released into the electrolyte to be combined with one electron at the negative electrode, which goes through an external circuit from the positive to negative electrode.The remaining compound in the LiGaO 2 electrode thus has to be oxidized to maintain the charge balance.
Ga as a group IIIA member can only have valence states as Ga 0 or Ga 3+ .−58 Thus, Ga 3+ is unlikely to change its valence state during the electrochemical oxidation reaction at high voltage, limiting the possible reaction product to Ga 3+ containing oxide compounds.However, the formation of Li− Ga−O ternary compounds with a lower Li-stoichiometry than LiGaO 2 , such as LiGa 5 O 8 , does not agree with thermodynamic considerations of the Li−Ga−O stabilities at high voltage either.−58 To form Ga 2 O 3 from 2 LiGaO 2 , 1/ 2 O 2 needs to be released to balance the reaction.This O 2 evolution reaction (OER) is instantaneously coupled to the decomposition of LiGaO 2 above 3.5 V vs Li/Li + and will readily oxidize the other compounds in the electrode.The proposed OER during reaction (1) agrees with our observations in SEM/EDX, where large oxidations in the region of the binder matrix was observed in the charged sample after 7 cycles and where areas with high concentration of Ga were surrounded by an O-rich periphery, while Ga itself could not be separated from O even at the length scale of SEM/EDX.Finally, STEM/EELS revealed that Ga 2 O 3 is present in the sample besides LiGaO 2 at the nanometer length scale, thus unambiguously proving that LiGaO 2 decomposes to Ga 2 O 3 at potentials above 3.5 V vs Li/Li + , and Li being extracted electrochemically from the system.
During discharge, measurable activity is detected below 2.5 V vs Li/Li + .The current increases steadily while going to lower potentials.Based on the previous observations for Ga 2 O 3 formation in reaction 1, the reaction 2 during discharge is expected to arise from the Ga 2 O 3 electrochemical conversion reaction, leading to metallic Ga and Li 2 O.The conversion reaction of Ga 2 O 3 is well-known in the literature and has been demonstrated to lead to subsequent reversible alloying reactions that take place between 0.0 and 1.0 V vs Li/Li + . 36his agrees to the observed reversibility of the electrochemical reaction in our experiments below 1.0 V vs Li/Li + , for comparison see also Figure S5, Supporting Information.
The ideal electrochemical Li−Ga alloying reaction shows distinct electrochemical reactions at 0.14 V vs Li/Li + during discharge and 0.78 and 0.86 V vs Li/Li + during charge, Figure S6, Supporting Information.While striking differences can still be seen in terms of polarization and reversibility of the pure Li−Ga alloying reaction, the agreement with the alloying potential window of Li−Ga is obvious.Furthermore, our data agree very well with the Ga 2 O 3 electrochemical activity, 36,37,47 in terms of shapes and reversibility of the observed CV curves (Figure S5, Supporting Information).Furthermore, the reaction product in reaction 3 is metallic Ga, which has been clearly identified to be present in the charged sample after 7 cycles via XPS, in addition to Ga 2 O 3 and LiGaO 2 .
The presence of LiGaO 2 in all ex situ samples, as evidenced by XRPD, Raman spectroscopy, XPS, and EELS implies that, for the chosen experimental setup, LiGaO 2 has not been fully activated during the recorded 7 cycles.This may be due to the rather large particle size of LiGaO 2 that limits the electrochemical reaction to the surface of the LiGaO 2 particles.Especially LiGaO 2 itself does not provide sufficient electronic conductivity to free up electrons upon decomposition of bulk LiGaO 2 in reaction 1.From the observed microstructural degradation through QPA based on the ex situ XRPD data, we can conclude that most of the activation of LiGaO 2 takes place in the first charge.During the first discharge, Ga 2 O 3 conversion is the major reaction, while in the subsequent charge and discharge cycles, the Li−Ga alloying and dealloying dominate.
The decomposition of LiGaO 2 at electrochemical potentials >3.5 V vs Li/Li + and the subsequent electrochemical reactions suggest that the presence of LiGaO 2 at grain boundaries of Ga:LLZO electrolytes can subject full cell ASSLMB with Ga:LLZO electrolytes to short circuiting during cycling.−3 Cycling these cells will easily decompose LiGaO 2 into Ga 2 O 3 .Since Ga 2 O 3 has an electrochemical potential higher than metallic Li, Li + will preferably react to Ga 2 O 3 to form Ga and Li 2 O instead of reducing to Li metal at the negative electrode during the discharging process.Therefore, short circuiting of the ASSLMB will happen through the LiGaO 2 phase even without physical contact to the metallic Li at the negative electrode.The experimental results thus highlight the importance of eliminating LiGaO 2 formation at the grain boundaries of Ga:LLZO to realize its ASSLMB application.

■ CONCLUSIONS
The study reports the electrochemical activity of LiGaO 2 , tested in a Li-ion type battery as positive electrode against Li metal as a negative electrode, with a Li + conducting liquid electrolyte.We could show that LiGaO 2 gets activated above 3.5 V vs Li/Li + , by decomposing to Ga 2 O 3 under OER and the extraction of Li + into the electrolyte, thus leading to the anodic reaction to Li metal at the negative electrode.The subsequent electrochemical reactions involve the reversible Li−Ga alloying and dealloying in the voltage range from 0.05 to 1 V vs Li/Li + with significant capacities of up to 200 mAh g −1 .The results have considerable implications for the use of full solid-state cells with Ga-doped garnet solid electrolytes, where LiGaO 2 is known to coexist with the garnet main phase.Especially for the cycling conditions in full cells, usually above 3 V vs Li/Li + , LiGaO 2 is expected to be activated and electrochemically reacted into Li−Ga alloy phases.Those reaction products will subject the solid electrolyte to cell failures through short circuits.The results highlight the importance of the knowledge-based design of solid electrolytes to realize highperformance solid-state batteries.

■ ASSOCIATED CONTENT
* sı Supporting Information

Figure 3 .
Figure 3. Results of the constant current (CC) + constant potential (CP) charge and discharge of LiGaO 2 against Li metal in an electrochemical test cell.(a) Cycling protocol with indicated OCV, CC, and CP periods; (b) potentials over capacities achieved with CC charge/discharge for the 1st to the 7th cycle; (c) achieved CC capacities in comparison to achieved CC + CP capacities as a function of cycle number; (d) efficiency as discharge capacity divided by charge capacity for CC and CC+CP as a function of cycle number; and (e) potential after each 20 h OCV period ("equilibrated" potential) as a function of cycle number.
for 20 h.It has an equilibrated potential of 0.22 V vs Li/Li + .D-1 is taken after one full cycle plus rest (oxidation to 4.5 V and reduction to 0.05 V vs Li/Li + and another oxidation to 2.75 V vs Li/Li + and resting at 3.0 V vs Li/Li+ for 20 h).It has an equilibrated potential of 2.5 V vs Li/Li + .B-7 is taken at the position of B-1 but after 7 additional cycles of charge− discharge.It has an equilibrated potential of 1.5 V vs Li/Li + .C-7 is taken at the position of C-1 but after 7 additional cycles of charge−discharge.It has an equilibrated potential of 0.25 V vs Li/Li + .

Figure 5 .
Figure 5. Results of the QPA based on XRPD data of the ex situ samples A-1 to C-7 (a) experimental, calculated, and difference for the fits in the fitting ranges 17−80°2θ; (b) evolution of refined LiGaO 2 lattice parameters over the sample series; and (c) evolution of refined crystallite sizes of the sample series.

Figure 6 .
Figure 6.Results of principal component analysis from micro-Raman mappings for the samples (a) B-1 (first charge to 4.5 V vs Li/Li + ), (b) D-1 (first full cycle), and (c) B-7 after charging to 4.5 V vs Li/Li + in the 7th cycle.Each color represents an identified individual component through the principal component algorithm.Gray areas are out of focus or have intensities that are too low.For each component, a representative individual spectrum is shown: (d) LiGaO 2 , (e) decomposed LiGaO 2 or Ga-oxide, (f) low intensity area/decomposed LiGaO 2 , (g) unassigned, and (h) low intensity area/decomposed LiGaO 2 .

Figure 7 .
Figure 7. SEM and correlative EDX mappings for (a) top view of the B-7 sample (left accelerating voltage 5 kV, right accelerating voltage 20 kV) and (b) cross section of the B-7 sample (accelerating voltage 5 kV).

Figure 8 .
Figure 8. XPS spectra for the B-7 sample and metallic Ga reference sample.Panels a and b are Li(1s) and Ga(3d) spectra for the B-7 sample; panels c and d are Ga(3d) and Li(1s) spectra for metallic Ga samples.Panels e and f are spectra belong to O(2s) for B-7 and metallic Ga, respectively.(g) XPS survey for both B-7 and metallic Ga samples.
to O and surrounded by an O-rich periphery, as evidenced by the Ga−L and O−K signals (Figure S3, Supporting Information, provides further EDX maps showing similar features on a different area of the sample).Interestingly, the areas where binder is present (F and P rich) show significant signals for the O−K, suggesting that the binder became heavily oxidized or new oxide products have been formed among the matrix.X-Ray Photoelectron Spectroscopy and Electron Energy Loss Spectroscopy.Synchrotronic XPS measurement for LiGaO 2 at seventh cycle (charged state, B-7) and metallic Ga (as a reference) were performed in the 14A beamline of the Taiwan light source.For the LiGaO 2 sample (B-7), Li(1s) spectra indicate the presence of Li 2 O and LiGaO 2 from Li−O at around 57 eV and possible Ga 2 O 3 / LiGaO 2 from Ga−O at 58.27 eV 49−51 as indicated in Figure 8a−c.However, in Figure Furthermore, the O(1s) spectra from Figure 8e for the LiGaO 2 sample shown two peaks associated with C−O (534.06 eV) 5 from the conductive carbon or electrolyte and Ga−O (532.56 eV) peak from the LiGaO 2 electrode sample, whereas the metallic Ga samples shown one intense peak at around 532.56 eV from Ga−O 52 as indicated in Figure 8f.Here, based on O(1s) spectra, it is hard to identify LiGaO 2 or Ga 2 O 3 because of the similarity in their binding energy.The Li(1s) and Ga(3d) spectra can be used as solid evidence for the presence of LiGaO 2 in the B-7 sample, which agrees to our previous observations from PXRD, Raman, and electrochemical analyses where the presence of LiGaO 2 was confirmed too.The XPS survey for both LiGaO 2 and metallic Ga included for comparison purpose as demonstrated in Figure 8g.Ga 2 O 3 cannot be separated unambiguously from LiGaO 2 through XPS, due to the coexistence of LiGaO 2 and their

Figure 9 .
Figure 9. EELS spectra of the B-7 sample in different areas.(a) ADF-STEM image of the sample.(b) Low-loss EELS spectra, and (c) ionization edges corresponding to the O−K and Ga−L edges as indicated of selected area in panel a.The near-edge fine structure (ELNES) of Ga−L is enlarged (inset).(d) ADF-STEM image of the sample.(e) Low-loss spectra of the surface region, it shows the presence of Li.The ELNES of Li−K is enlarged (inset, f) corresponding O−K and Ga−L edges for the selected area as shown in panel d.The ELNES of the Ga−L peak is enlarged (inset).
Figure 9c displays the O K-edge and Ga L-edge, enabling us to identify the composition of this region as Ga 2 O 3 , 53,54 thus proving Ga 2 O 3 formation from LiGaO 2 decomposition above 3.5 V vs Li/Li + .Furthermore, in the surface area, as shown in Figure 9d−f, LiGaO 2 was detected from its Li−K, O−K, and Ga−L signals.This proves the existence of Ga 2 O 3 besides LiGaO 2 on a length scale of a few hundred nm.Derived Reaction Mechanism for LiGaO 2 Electrochemical Activity.Based on our observations from electrochemical analysis and ex situ Raman, XRPD, SEM/EDX, STEM/EELS, and synchrotron XPS analyses, we derive the following reaction mechanism for the electrochemical activity of LiGaO 2 (Figure10).

Figure 10 .
Figure 10.Overview of the identified reaction mechanism for LiGaO 2 from 0.05 to 4.5 V vs Li/Li + .